The invention relates generally to a film cassette.
More particularly, the invention relates to a film cassette for sheet film.
A known cassette for sheet film has upper and lower cassette portions which cooperate to define a film compartment. The upper cassette portion has a window via which the film is exposed. A shield for blocking the window and thus preventing undesired exposure of the film is slidably received in a slot provided in the upper cassette portion. In addition to the shield, the upper and lower cassette portions have cooperating sealing portions which prevent light from entering the film compartment.
Film cassettes of this type are known as CRT cassettes and are used in cameras, primarily multiple image cameras, which are designed to accept film in the form of sheets.
Multiple image cameras are used to obtain reproductions of differing size of images formed on high resolution screens. Many reproductions (25 or more) may be produced on a single sheet of film or, alternatively, a single sheet of film may be used for a single reproduction. It is even possible to obtain many reproductions on a single sheet of film with a single exposure. These multiple image cameras are used mainly for medical purposes.
A film cassette for use in a multiple image camera is known from the German Pat. No. 27 50 559. The film cassette is normally placed in the camera with the window facing downwards. After removal of the shield, the film is exposed by light travelling towards it from below.
A movable pressure plate is provided to keep the film flat. The pressure plate is urged towards the window by means of resilient strips or fingers which are actuated via follower controls, e.g. cam-operated controls. It has been found, however, that the film sags in the middle so that the reproductions are blurry. This is particularly true for large sheets of film. The sagging results from the fact that the pressure plate does not press against the frame of the window uniformly. This enables air to penetrate the area between the film and the pressure plate, which, in turn, leads to sagging of the film.
Furthermore, the known cassette referred to above is designed exclusively for automatic loading and unloading. This requires an extremely complicated mechanism to release the pressure which the pressure plate applies to the film and to swing away a flap which covers a slit for insertion and removal of the film. In addition, the exposed film must be removed from, and a fresh sheet of film inserted into, the cassette via the slit. Not only is the mechanism which performs these functions expensive but the introduction of the film into the slit is complicated since the latter is small.